A mounting structure constructed by mounting an electronic component on a wiring board has conventionally been used for electronic equipment.
As an example of the wiring board, there is a construction comprising an inorganic insulating layer (ceramic layer) and an electrically-conductive layer (nickel thin layer) disposed on the inorganic insulating layer (for example, refer to Japanese Unexamined Patent Publication JP-A 4-122087 (1992)).
In such a wiring board, to provide electrical connection between electrically-conductive layers disposed above and below the inorganic insulating layer, there may be a case where a through hole is formed so as to penetrate the insulating layer in a thickness direction thereof, and a through conductor is formed so as to adhere to the inner wall of the through hole while being electrically connected to the electrically-conductive layers.
In this case, when the wiring board is subjected to heat at the time of installation or actuation of an electronic component, due to the difference in thermal expansion coefficient between the inorganic insulating layer and the through conductor in the thickness direction, a thermal stress is applied between the through conductor and the inner wall of the through hole, thus causing the through conductor to separate from the inner wall of the through hole. When the separation is increased and reaches the junction between the through conductor and the electrically-conductive layer, cracking may occur in the junction. This leads to a break in the wiring of the wiring board, thus causing a decline in the electrical reliability of the wiring board.
Hence, provision of a wiring board having excellent electrical reliability is demanded.
An object of the invention is to provide a wiring board having excellent electrical reliability, and a mounting structure using the wiring board.